The answer to this can be found in Chapter 15 of Guns, Germs, and Steel. Specifically, it can be found on page 305 of the paperback edition of the book. The answer is human muscle power.

In this chapter, Diamond is trying to explain why New Guinea and Australia developed (or failed to develop) as they did. He is trying to explain why New Guinea failed to modernize even though it had food production from a relatively early time. Diamond gives three reasons why this happened, all of which have to do with geography or biology.

The factor that is relevant to this question is the lack of animals. Diamond discusses the fact that New Guinea had very few animals of any size. Therefore, the only source of power was the power that human beings could create on their own. As Diamond says:

Since neither pigs nor chickens can be harnessed to pull carts, highlanders remained without sources of power other than human muscle power…

Thus, the answer to this is that human muscle power was the only source of power available to the people of New Guinea.